If you’re new to graphic or web design, you may find it confusing when you use an image editor and see some of the various color models and modes available. This post aims to briefly explain the theory behind models in Photoshop and why you would choose one particular model over the other.

What is a color model? A color model is simply a way to define color. A model describes how color will appear on the computer screen or on paper. Three popular color models are:

1. The CMYK model is used for print work and it describes colors based on their percentage of Cyan, Magenta, Yellow and Black. These four colors are used by commercial printers and bureaus and you may also find that your home printer uses these colors too. These four colors are needed to reproduce full color artwork in magazines, books and brochures. By combining Cyan, Magenta, Yellow and Black on paper in varying percentages, the illusion of lots of colors is created.

CMYK is known as a “subtractive” color model. White is the natural color of the paper or other background, while black results from a full combination of colored inks.

2. The RGB model is used when working with screen based designs. A value between 0 and 255 is assigned to each of the light colors, Red, Green and Blue. So for example, if you wanted to create a purely blue color, Red would have a value of 0, Green would have a value of 0 and Blue would have a value of 255 (pure blue). To create black, Red, Green and Blue would each have a value of 0 and to create white, each would have a value of 255. RGB is known as an “additive” model and is the opposite of the subtractive color model.

In this situation, when we talk of “value” of color, we’re referring to the strength of the colors in relation to each other. In the image below, you can see a photograph separated into its red, green and blue elements.

3. The Lab color model is a slightly more complex beast. It is made up of three components – the lightness component (L) ranging from 0 to 100, the “a” component comes from the green-red axis in the Adobe Color Picker, and the “b” component which comes from the blue-yellow axis in the Adobe Color Picker. Both “a” and “b” can range from +127 to –128. When Photoshop is converting from one model to another, it uses Lab as the intermediate color model.

So, after all that which model should you use?

If you know that your work is being sent to a commercial printer, then it’s a good idea to start your document in CMYK mode. Otherwise it’s safe to say that you can work in RGB for almost any other non-printing project. It is possible to convert from one color model to another but as RGB and CMYK spaces are both device-dependent spaces, the conversion formula is not that simple and requires a color management system (such as Photoshop or other graphics editor) using color profiles and that is why conversions are rarely exact. If you do need to convert from one color model to the other, it’s just a matter of choosing Image > Mode and then picking the one you need.